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Extend this to Hard Mode dungeons and you'll get my vote, OP.
"Insanity Defense" - Defeat Siren without letting her cast Lunatic Song.
"A-V-E-R..." - Defeat Avere Bravearm after your party has been hit with an almost full-charge Red Wedding three times.
"Immaculate" - Defeat Giruveganaus without any member of your party getting hit with Infirmity.
The last one would be the group breaker. The one that would cause people to rage-quit mid-fight when someone gets hit with Infirmity.
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Bumping in hope other have suggestions for dungeon/raid achievements and titles.